Certain populations of the mosquitofish, Gambusia affinis, are highly resistant to several chlorinated alicyclic insecticides, including cyclodienes. The patterns of resistance in these fish are very similar to those in cyclodiene-resistant insect populations. Resistance to cyclodiene insecticides in insects is inherited at a single locus with semi-dominant alleles. We propose to test the hypothesis that resistance in the mosquitofish, a vertebrate, is inherited in a similar manner. To test the hypothesis that insecticide resistance is inherited at a single locus, a genetic analysis of the inheritance will be conducted through a program of genetic crosses. Resistant and susceptible breeding fish populations will be established in the laboratory. Progenies of these populations and reciprocal F1 crosses between them will be bioassayed using the cyclodiene endrin to establish the resistance phenotypes and dominance of resistance in heterozygotes. Appropriate backcrosses will then be produced and similarly bioassayed to determine the number of loci involved. From these data, the inheritance of resistance will be deduced. The proposed research will provide, for the first time, a basic understanding of the mode of inheritance of cyclodiene resistance in a vertebrate. This might be useful in investigating the mode of action of cyclodiene insecticides, which, in itself, may aid in understanding membrane function. Information of the inheritance of resistance should improve the feasibility of using mosquitofish for control of disease vectors in agricultural situations. In general, this research will provide a more basic understanding of the responses of non-target vertebrate populations to a major group of insecticides and be useful in predicting the responses of vertebrate animal populations to other xenobiotics.